In the well logging industry, it is necessary for a sonde lowered on a logging cable into a well to operate at temperatures which approach 200.degree. C. While the precise temperature will vary depending upon well depth and other circumstances, there is a marked temperature increase acting on circuit components enclosed in the sonde. This particularly has substantial impact on power supply components. Particularly troublesome are DC to DC converters which utilize field effect transistors to accomplish switching. With increased temperature, the switching speed of the FET decreases. Moreover, heat dissipation from the FET increases yielding thermal runaway in the FET.
The temperature increase impacts components other than the FETs. Distributed circuit resistance values increase with temperature and this is a factor in power supply transformers. While protective circuit components can be added (as an example, clamping diodes which limit voltage levels in the circuit), this is a difficult requirement in obtaining a DC to DC converter suitable for installation in a sonde. This inevitably requires that the circuit operate at high temperatures, always subject to the possibility of thermal runaway. The apparatus of this disclosure is a DC to DC converter circuit having an improved arrangement of components which reduce the performance requirements on the FET switches and the other circuit components. One mode of improving component performance is the incorporation of a high speed FET switch to increase the frequency of switching, thereby reducing ripple filtering problems, smoothing the operation of the rectifier system and providing other benefits. The switching time of the FET is stretched on exposure to high temperature. This apparatus enables a slower switching speed and yet it is accomplished with a transformer of reduced size. This loads the switch with a different impedance. It also reduces voltage stress on transformer winding voltage flyback. As the flyback peaks increase, a more expensive and heavy duty design is ordinarily required. The DC to DC converter of this disclosure enables operation at high temperatures more readily. The apparatus utilizes a coupling transformer having a primary terminal connected to the output of a full wave rectifier bridge connected to the transformer secondary. By this arrangement, flyback occurring in the transformer circuit is reduced and transformer power dissipation is also reduced. A wider range of output currents can be drawn from the system. Moreover, it can be driven at lower switching frequencies and still provide a relatively smooth DC output.